The use of a screw rotating inside a close-fitting cylinder to move material forward has had many commercial applications. The ability of a particular screw to move the material forward is greatly dependent on the nature of the material being moved. It is possible to use a screw to pump water, syrups, soaps, molten polymers, elastomers and even solid metals, but the apparatus to do each of these operations must be built with the limitations and the properties of the pumped material clearly in mind. There is no such thing as a "universal" extruder which handles all types of material well.
The first large-scale commercial application of screw extruders came with the development of rubber tires for automobiles. The processing of uncured rubber required mechanical working, incorporation of fillers and vulcanizing agents, control of temperature and the extrusion of the well-mixed compound into a strip of desired cross section for further processing. The efficiency of producing the right degree of each of these several goals was related to the design of the screw and barrel as well as to the speed of rotation of the screw. As a result, large numbers of improvements were made to increase the utility of the extruder and to make feasible a high rate of production without loss of vital control features such as temperature and pressure. It is important to point out that all rubber compositions in the uncured state are soft and "tacky," that is they readily adhere to almost everything, but especially to smooth metal surfaces such as found in the screw and barrel of rubber extruders and to itself.
In the early days of the plastics industry, most of the material produced was some form of nitrocellulose which was very dangerous to handle in the solid state. While it could be softened somewhat with heat, excessive local heat would cause the ignition and explosion of the material. For this reason, the nitrocellulose and plasticizer (such as camphor) were combined with a solvent (such as ethyl alcohol) to give a mix that could be processed with reasonable safety into a semi-finished article which was then put into usable condition by seasoning out the solvent. The solvent-containing mix was quite similar in its properties to an uncured rubber mix, so that it proved practical to use a screw and barrel adapted from the rubber industry. One such application is shown in U.S. Pat. No. 2,146,532 that shows the use of a screw to remove plastic material from an evacuated chamber.
From 1930 to 1940, many synthetic resins were developed. This was the time for the separation of the plastics industry into two distinct branches; the elastomers, comprising rubber and synthetic rubber-like materials, and the synthetic resin materials, characterized by being relatively hard and slippery at room temperature, and being so thermally stable that they could be melted to a low viscosity (compared to rubber) liquid that could be easily formed into articles of desired shape and dimension by simply cooling below the melting point. Among these synthetic polymers should be mentioned polyethylene, polypropylene, polymethyl methacrylate, poly vinyl chloride, polyoxy methylene, nylon, polyethylene terephthalate and polytetrafluoroethylene. All of these materials are very different from rubber and other elastomeric materials in hardness and coefficient of friction toward metals and toward itself. Unlike rubber, they are sharp melting or have a very narrow temperature range between a hard solid and a low viscosity fluid. Also, some are very sensitive to decomposition from heat, so that local overheating must be avoided in any processing step.
One of the first applications of an extrusion screw to molten nylon is described in my U.S. Pat. No. 2,295,942 that shows how a very shallow threaded screw can be used to give uniform delivery of a molasses-like liquid against considerable back-pressure of a filter and die.
The use of this type of screw thread was extended to include the melting of nylon polymer chips in the portion of the extruder adjacent to the feed throat. It was soon found that this type of screw and barrel also gave excellent results with polyethylene resins, and the design was freely offered by duPont to the makers of extrusion equipment. The introduction of the screw and barrel designed specifically for the new class of synthetic resins was followed by a tremendous growth in the industry and in the industry supplying the new type of extruder.
The rapid growth of the extrusion business made it expedient to conduct an engineering study of the process in order to optimize the efficiency of the equipment. These studies were reported through meetings and publications of the Society of Plastics Engineers and did much to call attention to the subtle variables which affect the performance of an extruder. Several U.S. patents were issued covering some of these improvements.
Most of these prior art developments involve the design of the screw element but some of them relate to the barrel or cylinder in which the screw rotates. An example of the latter is the use of fluting or threading inside the barrel at the feed end of the extruder. The use of longer barrels and screws assist in maintaining a uniform rate of resin output, however, at the same time temperature control becomes almost unmanageable with resins having high viscosity. The use of shallow screws provides better control of output uniformity, but appreciably reduces the amount of resin output. Similarly, the use of lower lead angles for the thread of the screw provides a more positive delivery at the expense of resin output. For these and other reasons, it has been found profitable to use a computer to determine the optimum design for an extruder for a particular resin, since each resin has its own properties of hardness, viscosity, coefficient of friction, etc.
As a result of the many studies on the effects of various types of screws and barrels, we find the type of extruder in common use for materials of the type of nylon, polyethylene, polyoxymethylene, etc., to be one of shallow depth screw, a single lead screw with lead roughly equal to screw diameter, a compression ratio from 1 to 2.5 or more, and a barrel length from 15 to 30 times the diameter of the bore of the barrel. Such an extruder gives an excellent performance when used with such materials as polyethylene of low or medium molecular weight, polypropylene and polymethacrylate. However, the physical properties of unmelted nylon, polyesters and high molecular weight polyethylene and polytetrafluoroethylene are such that the performance is erratic, and severe limitations are put upon the ability to operate the extruder at commercially useful rates. This appears to be due to the very low coefficient of friction of the polymer toward the metal wall of the extruder barrel. A study was made of ways to increase the grip between the polymer and the inner wall of the extruder barrel without hurting the ability of the extruder to give high output, uniform delivery and high pressure without appreciable loss of output.
In an attempt to improve the performance of the type of extruder described above, it was thought that narrow, shallow grooves cut into the inner surface of the barrel in a longitudinal direction would greatly increase the grip between the barrel wall and the polymeric material. Such a barrel was made with six shallow grooves (lengthwise of the barrel) approximately 1/8 inch wide and 1/16 inch deep. Upon testing with polytetrafluoroethylene, it was found that the delivery of molten polymer from the extruder was excellent, so long as no restriction was placed on the flow from the exit of the extruder. When the outlet was restricted to build pressure, the output dropped rapidly so that at modest pressure of 2,000 to 3,000 psi, all output went to zero. Since the main object of building this extruder was to obtain a practical device for delivering through a filter (normal back-pressure of approximately 4,000 psi), it was apparent that the desired apparatus was not yet available.
After some months of trying to think of a way of gripping the wall of the barrel by the polymer, the idea came to me to make the barrel groove of the same hand, same lead and of a width less than the width of the land of the screw with which the barrel is associated. In this way, no openings would be made for the molten polymer to flow over the rib of the screw from the front (higher pressure) side to the rear (lower pressure) side, and we would have for the first time a barrel with grooves cut into it and still no place for a polymer to flow over the rib and thus destroy its pressure-building ability.
Of course, there was no good reason to believe that such a groove would help the barrel to grip the polymer and prevent the polymer from turning with the screw and thus not move forward in the desired manner. I discussed this feature (confidentially) with a highly respected plastic extrusion engineer and after consideration, he assured me that "such a groove would not help the performance -- to be effective the barrel groove must be of opposite hand."
In spite of this discouraging appraisal of the likelihood of success, I decided to build and test such an extruder barrel for a 2-inch extruder which normally operates with a shallow screw with a 2-inch lead, a length of 15 to 30 diameters and a compression ratio of 2 to 3. The screw was right hand; the land of the screw about 1/4 inch.
The tests were successful and the machine put into production.